The present invention relates to a semiconductor device and a method for fabricating the semiconductor device, more specifically a semiconductor device which can increase the carrier mobility while suppressing the short channel effect, and a method for fabricating the semiconductor device.
Recently, to realize high speed and micronization of MOSFETs it is noted to use SOI (Silicon On Insulator) substrates.
A proposed semiconductor device using an SOI substrate will be explained with reference to FIG. 27. FIG. 27 is a sectional view of the proposed semiconductor device.
As shown in FIG. 27, a semiconductor layer of Si is formed on a substrate 110 of Si with an insulation film 112 formed therebetween. The substrate 110 is a base substrate for the SOI substrate 108. The insulation layer 112 is a buried oxide film of the SOI substrate 108. The semiconductor layer 114 is an Si layer formed on the buried oxide film of the SOI substrate 108. A gate electrode 122 is formed on the semiconductor layer 114 with the gate insulation film 120 formed therebetween. A source/drain diffused layer 124 is formed in the semiconductor layer 114 on both sides of the gate electrode 122.
In the proposed semiconductor device using the SOI substrate, because of the insulation film 112 formed between the lower surface of the source/drain diffused layer 124 and the silicon substrate 110, a parasitic capacitance between the source/drain diffused layer 124 and the silicon substrate 110 can be small. The SOI substrate permits the source/drain diffused layer to be formed by shallow junction.
Then, recently, the semiconductor device is increasingly micronized. As the semiconductor devices are more micronized, the semiconductor layer 114 is increasingly thinned so as to prevent the short channel effect.
However, as the semiconductor layer 114 is increasingly thinned, the carrier surface scattering and the phonon scattering are increased in the interface between the semiconductor layer 114 and the buried diffused layer 112, which lower a carrier mobility (Reference: The division of the Japan Society of Applied Physics, Silicon Technology, No. 35, 22th Jan. 2002, pp. 88-93). The decrease of the carrier mobility has been a barrier to higher speed of the semiconductor device.